1. Field of the Invention
The present invention relates to an apparatus to process an arbitrarily polarized source signal. In particular, the invention relates to an apparatus to separate an arbitrarily polarized signal into two components and then align the polarizations of the components for processing. Exemplary applications include mixing the polarization aligned signal components in a nonlinear crystal with a pump signal having a predetermined polarization.
2. Description of Related Art
Nonlinear wavelength conversion can be used to measure very high-speed signals in an optical sampling oscilloscope. Nonlinear conversion crystals typically require specific polarizations of the source signal and the pump signal, and the crystals produce a sum frequency generation (SFG) signal having the same polarization orientation. Whereas the pump signal, being part of the instrument, can always be polarized correctly, the source signal (i.e., the user's input signal) may have unknown polarization. Polarization controllers based on feedback have been used in the past but these are capable of polarization adjustments of at best several kilohertz bandwidth. Polarization interleaving has been employed so that the polarization changes at the bit rate, necessitating tens to hundreds of gigahertz of polarization-acquisition bandwidth. It is desirable for an optical sampling oscilloscope to simply display the reconstructed signal power from the source signal independent of the input polarization.
Use of a polarization beam splitter that is polarized on a 45 degree axis with respect to the principal axis of the polarization beam splitter to split a pump signal into two channels is known. In this way, 50% of the pump power passes into each of the two different channels, each channel being characterized by a polarization orthogonal to the other. The source signal is also passed through the polarization beam splitter, and the two polarization components of the arbitrarily polarized source signal are split apart. Each of these two channels are passed through a corresponding nonlinear crystal to produce two corresponding mixed signals. However, this results in a 50% reduction (i.e., 3 dB loss) in conversion efficiency due to the reduced intensity of the pump signal.
The use of two nonlinear crystals in series so as to not suffer a theoretical 3 dB penalty (i.e., due to splitting the pump signal) is also known. However, the two crystals add to the cost, there is a need to employ an optical component between the two crystals to correct for color and temporal dispersion, and there is a need to match the crystals, especially their temperatures.